I was raised a Jehovah's Witness, so I know all the "ins-and-outs" of their theology and belief system.
In my view, they can become a part of Mainstream Christianity and lose their cult status by making just a few changes for the better:
1: Stop their severe shunning practices. It's okay to avoid association with people you dislike or whom you believe are not a good influence on you. It's even okay for church elders to rebuke or even excommunicate someone for gross, flagrant sin. But don't try to cut off communication among family members, and don't disfellowship someone just for disagreeing with the way you interpret the Bible.
2: Stop excommunicating, punishing, or shaming members of their organization who choose to have a life-saving blood transfusion. It's okay to point out that you believe the Bible says to abstain from blood, (Acts 15) but you should allow each Christian to make a personal decision based on their conscience, not make it a mandatory law.
3: Stop saying that Christians will be destroyed by God if they don't join their organization. Acknowledge that Jesus can save people anywhere, as long as they put faith in Him, His sacrifice, resurrection, and His Father.
4: Stop saying that Christians must preach the Gospel to others in order to be saved. Romans 4 disagrees with this view.
5: Stop saying that Christians must attend meetings to be saved. Nowhere does the Bible say this.
If the Jehovah's Witnesses change just these 5 things, they can be considered a Mainstream Christian Religion, and no longer be a cult.
All their other beliefs can be reasonably obtained from the Bible, depending on your interpretation. In the last 10 years, they have abandoned most of their far-out prophecy interpretations and their interpretation of types/antitypes.
Other Unorthodox Views of Jehovah's Witnesses
The Trinity is false.
Jesus was the first creature made by God.
Jesus is Michael the Archangel.
Jesus returned invisibly in 1914 to become King.
The Holy Spirit is not a person, but is God's active force or energy.
Only 144,000 Christians in all of history will end up going to Heaven.
All other Christians who are saved will live on a Paradise Earth forever.
The Governing Body of Jehovah's Witnesses is similar to the 12 Apostles, and they are the sole channel of communication between Jesus and Christians today, to help them interpret the Bible.